This invention relates to a transmission power control method and a mobile terminal apparatus, and in particular relates to a transmission power control method for a mobile terminal when resuming transmission after transmission has been interrupted, and to a mobile transmission apparatus.
Specifications for radio communication systems using W-CDMA have been prepared by the 3rd Generation Partnership Project (3GPP), and actual services based on them are already in operation in many countries including Japan. FIG. 7 is a summary diagram of the configuration of a radio communication system. A 3GPP radio access system comprises a base station control apparatus (radio network controller) 1, base stations (node B) 2, 3, . . . , and mobile terminals (user equipment) 4, 5, . . . The base station control apparatus 1 is connected to the core network 6 via an Iu interface, and is connected to base stations 2, 3 via an Iub interface.
In a mobile communication system conforming to these 3GPP specifications, transmission power control is executed such that a prescribed error rate is obtained by base stations 2, 3 and mobile terminals 4, 5, and such that transmission power is not excessive. FIG. 8 is an explanatory diagram of such transmission power control (inner-loop power control), showing a case of transmission power control for a base station.
The spreading modulation portion 4a of a mobile terminal 4 uses a spreading code in spreading modulation of transmission data, and the power amplifier 4b amplifies signals obtained by orthogonal modulation, frequency conversion and the like after spreading modulation, and transmits the signals to the base station 2 from an antenna. The despreading portion 2a of the reception portion of the base station performs despreading processing of the received signals, and the demodulation portion 2b demodulates the received data. The SIR measurement portion 2c measures the power ratio SIR (Signal to Interference Power Ratio) of the received signal to the interference signal. The comparison portion 2d compares the target SIR and measured SIR, and if the measured SIR is higher than the target SIR creates a command (“down” command) to lower the transmission power using TPC (Transmission Power Control) bits, but if the measured SIR is lower than the target SIR creates a command (“up” command) to raise the transmission power using TPC bits. The target SIR is the SIR value necessary to obtain, for example, 10−3 (the occurrence of errors once every 1000 times), and is input to the comparison portion 2d from the target SIR setting portion 2e. The spreading modulation portion 2f performs spreading modulation of the transmission data and TPC bits. After spreading modulation, the base station 2 performs DA conversion, orthogonal modulation, frequency conversion, power amplification, and other processing, and transmits the data from an antenna to the mobile terminal 4. The despreading portion 4c of the mobile terminal 4 performs despreading processing of the signal received from the base station 2, the demodulation portion 4d demodulates the received data and TPC bits, and the transmission power of the power amplifier 4b is controlled according to the command indicated by the TPC bits.
FIG. 9 and FIG. 10 shows DPCH (Dedicated Physical Channel) frame formats of uplink (UL) and downlink (DL) physical channels, respectively, standardized by 3GPP. Downlink (DL) indicates the direction of data transmitted from the base station to the mobile terminal; uplink (UL) indicates the direction of data transmitted in the opposite direction, from the mobile terminal to the base station.
In FIG. 9, an uplink frame has a dedicated data channel (DPDCH, Dedicated Physical Data Channel) and a dedicated control channel (DPCCH, Dedicated Physical Control Channel). The dedicated data channel DPDCH transmits only transmission data; the dedicated control channel multiplexes and transmits pilot signals, TPC bit information, and other control data. One uplink frame comprises 15 slots (slot #0 to slot #14) in 10 msec. Each of the slots in the dedicated control channel DPCCH comprises 10 bits; the symbol rate is fixed at 15 ksps, and pilot signals, transmission power control data TPC, transport format combination indicator TFCI, and feedback information FBI are transmitted.
In FIG. 10, the downlink frame comprises 15 slots #0 to #14, with one frame=10 msec; each slot has a dedicated physical data channel DPDCH which transmits a first data portion Data1 and a second data portion Data2, and a dedicated physical control channel DPCCH which transmits pilot signals, TPC, and TFCI, using time-division multiplexing.
To summarize the above, uplink power transmission control in current W-CDMA 3GPP systems is performed as follows. At the initiation of communication, the mobile terminal 4 initiates communication by an initial power which is based on a provisional value determined by regulation according to a communication startup sequence (open-loop control), and thereafter, said up/down control of the transmission power is performed based on TPC bits sent continuously from the base station 2 in each slot (closed-loop control). By this means, the SIR value, in the reception portion of the base station 2, of pilot signals transmitted by the mobile terminal 4 are made equal to a prescribed target SIR.
However, in a fourth-generation mobile communication system and other radio packet transmission systems, data packet transmission may not be executed continuously. It is anticipated that, from the standpoint of effective utilization of radio interval resources during periods (transmission interruption periods) in which data packet transmission is not performed, systems will be designed so as to reduce the transmission of control packets insofar as possible.
Transmission power control for uplink UL transmission is based on the pilot signal transmitted in the uplink direction. But in the above fourth-generation mobile communication system, the pilot signals may not be transmitted during transmission interruption periods. Consequently during transmission interruption periods when uplink packet transmission is not performed, uplink transmission power control is no longer continuous, and the problem arises that transmission power values may be inappropriate at the time of resumption of data packet transmission.
In order to perform transmission at an appropriate transmission power even when data packet transmission is resumed, a mobile terminal 4 must continue to send a pilot signal to the base station 2 in the uplink UL direction even when packets are not being transmitted (transmission interruption interval A), as shown in FIG. 11, and at the same time, must continue to receive TPC commands from the base station 2. However, this method has the problem that efficient utilization of the radio interval is not possible.
Further, it is conceivable that, at the time of data packet transmission resumption, a transmission power value stipulated by specifications may be used. However, there is the possibility that the power value may deviate greatly from the value actually required. In this case, if the transmission power value stipulated by specifications is smaller than the value actually required, there arises the possibility that at the time of resumption of data packet transmission, transmitted data packets cannot be properly received and decoded by the base station; conversely, if the value is larger than the value actually required, the possibility that the base station will succeed in reception and decoding is increased, but increased occurrence of interference due to excessive power becomes a problem.
There exists a transmission power determination method in which, when determining the transmission power value at the time of transmission initiation, parameters of an equation to calculate the transmission power are changed (See JP2000-332682A). In this transmission power determination method (Prior Art Method 1), when the transmission power is determined using the equationTransmission power=Immediately preceding transmission power+TPC×Δ (where Δ is an increase/decrease step width)
the increase/decrease step width Δ is changed according to the environment conditions and terminal movement speed, so that for example Δ is increased when the terminal movement speed is high.
However, in this Prior Art Method 1, the transmission power at the time of transmission resumption is not determined taking into account the communication states at the time of the start of transmission interruption and at the time of transmission resumption, and so the problem occurs that transmission is not performed at an appropriate transmission power when transmission is resumed. In particular, the longer the period from interruption to resumption, the lower is the reliability of the transmission power provided by the above equation.
There exists a transmission power determination method in which, when determining the transmission power value at the time of initiation of transmission, the transmission power immediately preceding transmission interruption is taken into account (See Tokuhyo 2002-535872 corresponding to International Publication No. W00/42717). In this transmission power determination method (Prior Art Method 2), when determining the transmission power P06 at the time of transmission resumption as shown in FIG. 12, the transmission power can be determined using the following equation, based on the transmission signal outputs (P01, P02, P03, P04, P05) used in certain fixed intervals (in the figure, slot #1 to slot #5) immediately preceding transmission interruption, or on transmission power command values (TPC1, TPC2, TPC3, TPC4, TPC5).P06=P05+PoffsetPoffset=f(P01,P02,P03,P04,P05) orPoffset=f(P01,P02,P03,P04,P05,TPC1,TPC2,TPC3,TPC4,TPC5)
Here f(•) denotes a function which takes • as a variable.
However, this method assumes that the transmission interruption period is short (for example, 5 to 6 ms or less), and uses the transmission powers or transmission power command values for fixed intervals immediately preceding transmission interruption, and so there is the problem that at the time of transmission resumption, transmission at an appropriate transmission power is not possible. In particular, the longer the period from interruption to resumption, the lower is the reliability of the transmission power given by the above equation.